Hydrophobic surfaces bind very weakly with water, which makes drops of water form beads on the surface. A hydrophobic surface is generally defined and is defined herein as that which has a contact angle greater than 90° with a drop of water. Hydrophobic materials that form hydrophobic surfaces include many well-known, commercially available polymers.
A super-hydrophobic surface is generally defined and defined herein as that which has a contact angle greater than 150° with a drop of water at normal ambient temperatures (about 25° C.). The lotus leaf surface, for example, is known to be naturally super-hydrophobic due to the texture of its waxy surface.
There are four known methods for making superhydrophobic materials. One method relates to forming flat surface arrays of vertically aligned PTFE coated carbon nanotubes. A second method is based on forming periodic arrays of pillars on a flat surface using microelectronics based photolithography. A third method involves self aligned polymer nanospheres. The fourth method relates to using porous or roughened fluorinated polymers as a superhydrophobic coating material. Such roughened polymers show increased hydrophobicity and are therefore sometimes referred to as being superhydrophobic. By the standard definition of superhydrophobic that requires a contact angle of >150 degrees with a drop of water, such polymers are either not superhydrophobic, or at best are just barely superhydrophobic.
In principle, any material can yield a superhydrophobic surface because the contact angle that is measured in an air atmosphere results from the water bead having exposure to a supporting surface that is partially air. In practice, the material is hydrophobic. Air displays a contact angle with a water bead of 180°. The other portion is a solid, which when a smooth continuous surface provides a lesser inherent contact angle with water, that can occupy various proportion of the surface depending on its inherent contact angle. This relationship between the contact angle on the partitioned superhydrophobic surface and the portion of the surface that is solid is given by Cassie's law for a single uniform solid material:cos θc=γ(cos θ+1)−1where θc is the contact angle with water observed for the super hydrophobic surface, γ is the fraction of the surface under the water bead that is solid, and θ is the contact angle displayed with water for a smooth continuous solid surface.
Articles with superhydrophobic surfaces resist moisture to the extent that soiling of the surface is difficult. This resistance results from water being efficiently shed from the surface, carrying with it readily dissolved and wetted particulates. Another application for such a superhydrophobic surface is for dramatically lowering the resistance to water flow at that surface. As the affinity of the surface for air or other gas is much higher than for water, the resistance can be primarily defined by the viscosity of the water with air or other gas rather than the solid surface.
Superhydrophobic powders have the potential to improve a variety of existing technologies profoundly and allow the development of novel technologies. Superhydrophobic powders could be used in a wide variety of structures and applications. Identification of appropriate powder substrates with appropriate surface features, and methods of rendering the surface of those features sufficiently hydrophobic, are needed to yield powders with contact angles in excess of 150°.